The paper mentioned above describes the construction of various star configurations and also the advantages of lower losses of the star configuration compared with the bus configuration. A star configuration is therefore being used to an ever increasing extent in LAN networks.
In the paper mentioned, reflective star devices are given preference over transmission star devices since, in the first-named devices, the number of glass fibres, and therefore the components associated therewith, is reduced by a factor of two compared with the transmission star device. Instead of separate inputs and outputs for a transmitting and a receiving device, the reflective star device has ports which each serve both to input and output the signals to the reflective star device from a transmitting and receiving device. The various connections could in that case be provided with diplexers for separating the transmitted and received signals.
The known reflective star device is made up of a number of star elements, for example 3 dB coupling devices, transmission star elements or reflective star elements which distribute a signal fed to a port of such a star element over the other ports thereof. Reflectors are connected to a number of ports of the elements to create reflection points.
If a signal is fed to one of the ports of the reflective star device, said signal is distributed over the other ports thereof, but a reflection signal is also presented via the feed port to the transmitting and receiving device connected thereto. It is generally known that certain lasers are extremely sensitive to externally injected light, in particular if said light has a frequency near the characteristic optical oscillation frequency. In addition, such lasers are especially sensitive to reflections in their own polarisation direction and less sensitive to reflections having a perpendicular polarisation. In a conventional reflective star device, the signal also reflected to its own port may have any random polarisation.
One of the standard solutions used is to couple lasers badly into the glass fibre, with the disadvantage of a large power loss. Another standard solution is to use optical isolators for all the lasers, with the result that a large number of additional components are necessary, and in particular one optical isolator for each port.